1. Field of the Invention
The present invention relates to a catalyst for a polymer electrolyte fuel cell. In particular, the present invention relates to a catalyst useful for use in the air electrode of a polymer electrolyte fuel cell.
2. Description of the Related Art
A fuel cell, in particular, a polymer electrolyte fuel cell is highly expected as a next-generation power generating system, and has advantages of being low in working temperature and compact. Because of such advantages, polymer electrolyte fuel cells are regarded as promising as household and automobile power supplies. A polymer electrolyte fuel cell has a laminate structure consisting of a hydrogen electrode and an air electrode, and a polymer electrolyte membrane sandwiched between these electrodes. A hydrogen-containing fuel is fed to the hydrogen electrode, and oxygen or air is fed to the air electrode, and electric power is taken out on the basis of the oxidation reaction and the reduction reaction occurring in the respective electrodes. To each of both electrodes, a mixture composed of a catalyst to promote the electrochemical reaction and a solid electrolyte is generally applied.
As the catalysts constituting the electrodes, catalysts supporting precious metals as catalytic metals, in particular, platinum catalysts supporting platinum are widely used. The reasons for the use of the platinum catalysts as the catalysts for fuel cells reside in the activity of the platinum catalysts. Specifically, the foregoing reasons are ascribable to the fact that the platinum catalysts have high activity in promoting the electrode reactions in both of the fuel electrode and the hydrogen electrode.
As the recent popularization of fuel cells has become realistic, the catalyst for the polymer electrolyte fuel cell is required not only to be excellent in activity but also to be improved in various properties.
Examples of the requirements for the improvement of the catalytic properties include the improvement of the durability, namely, the improvement of the activity sustainability over a long period of time. Catalysts inevitably undergo the activity degradation occurring with the passage of time; however, the extension of the time until the deactivation can be said to be essential for the purpose of practical application of fuel cells. In this connection, as a method for improving the durability of the catalyst for a fuel cell, the method described in Patent Document 1 may be cited. The catalyst described in Patent Document 1 is a platinum supporting platinum catalyst subjected to heat treatment (annealing treatment) in such a way that the platinum particle size is regulated to be a predetermined particle size.